PKCS#7: Fix panic when referring to the empty AKID when DEBUG defined

This fix resolves the following kernel panic if an empty or missing
AuthorityKeyIdentifier is encountered and DEBUG is defined in
pkcs7_verify.c.

[  459.041989] PKEY: <==public_key_verify_signature() = 0
[  459.041993] PKCS7: Verified signature 1
[  459.041995] PKCS7: ==> pkcs7_verify_sig_chain()
[  459.041999] PKCS7: verify Sample DB Certificate for SCP: 01
[  459.042002] PKCS7: - issuer Sample KEK Certificate for SCP
[  459.042014] BUG: unable to handle kernel NULL pointer dereference at           (null)
[  459.042135] IP: [<ffffffff813e7b4c>] pkcs7_verify+0x72c/0x7f0
[  459.042217] PGD 739e6067 PUD 77719067 PMD 0
[  459.042286] Oops: 0000 [#1] PREEMPT SMP
[  459.042328] Modules linked in:
[  459.042368] CPU: 0 PID: 474 Comm: kexec Not tainted 4.7.0-rc7-WR8.0.0.0_standard+ #18
[  459.042462] Hardware name: To be filled by O.E.M. To be filled by O.E.M./Aptio CRB, BIOS 5.6.5 10/09/2014
[  459.042586] task: ffff880073a50000 ti: ffff8800738e8000 task.ti: ffff8800738e8000
[  459.042675] RIP: 0010:[<ffffffff813e7b4c>]  [<ffffffff813e7b4c>] pkcs7_verify+0x72c/0x7f0
[  459.042784] RSP: 0018:ffff8800738ebd58  EFLAGS: 00010246
[  459.042845] RAX: 0000000000000000 RBX: ffff880076b7da80 RCX: 0000000000000006
[  459.042929] RDX: 0000000000000001 RSI: ffffffff81c85001 RDI: ffffffff81ca00a9
[  459.043014] RBP: ffff8800738ebd98 R08: 0000000000000400 R09: ffff8800788a304c
[  459.043098] R10: 0000000000000000 R11: 00000000000060ca R12: ffff8800769a2bc0
[  459.043182] R13: ffff880077358300 R14: 0000000000000000 R15: ffff8800769a2dc0
[  459.043268] FS:  00007f24cc741700(0000) GS:ffff880074e00000(0000) knlGS:0000000000000000
[  459.043365] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  459.043431] CR2: 0000000000000000 CR3: 0000000073a36000 CR4: 00000000001006f0
[  459.043514] Stack:
[  459.043530]  0000000000000000 ffffffbf00000020 31ffffff813e68b0 0000000000000002
[  459.043644]  ffff8800769a2bc0 0000000000000000 00000000007197b8 0000000000000002
[  459.043756]  ffff8800738ebdd8 ffffffff81153fb1 0000000000000000 0000000000000000
[  459.043869] Call Trace:
[  459.043898]  [<ffffffff81153fb1>] verify_pkcs7_signature+0x61/0x140
[  459.043974]  [<ffffffff813e7f0b>] verify_pefile_signature+0x2cb/0x830
[  459.044052]  [<ffffffff813e8470>] ? verify_pefile_signature+0x830/0x830
[  459.044134]  [<ffffffff81048e25>] bzImage64_verify_sig+0x15/0x20
[  459.046332]  [<ffffffff81046e09>] arch_kexec_kernel_verify_sig+0x29/0x40
[  459.048552]  [<ffffffff810f10e4>] SyS_kexec_file_load+0x1f4/0x6c0
[  459.050768]  [<ffffffff81050e36>] ? __do_page_fault+0x1b6/0x550
[  459.052996]  [<ffffffff8199241f>] entry_SYSCALL_64_fastpath+0x17/0x93
[  459.055242] Code: e8 0a d6 ff ff 85 c0 0f 88 7a fb ff ff 4d 39 fd 4d 89 7d 08 74 45 4d 89 fd e9 14 fe ff ff 4d 8b 76 08 31 c0 48 c7 c7 a9 00 ca 81 <41> 0f b7 36 49 8d 56 02 e8 d0 91 d6 ff 4d 8b 3c 24 4d 85 ff 0f
[  459.060535] RIP  [<ffffffff813e7b4c>] pkcs7_verify+0x72c/0x7f0
[  459.063040]  RSP <ffff8800738ebd58>
[  459.065456] CR2: 0000000000000000
[  459.075998] ---[ end trace c15f0e897cda28dc ]---

Signed-off-by: Lans Zhang <jia.zhang@windriver.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
cc: linux-crypto@vger.kernel.org
cc: kexec@lists.infradead.org
Signed-off-by: James Morris <james.l.morris@oracle.com>